This invention relates to an image forming apparatus capable of performing a copying operation while inserting a tab sheet into a specified position (between specified pages) of a copied material which is stackingly discharged onto a discharge tray and, particularly to a transport of a tab sheet to be inserted.
Along with a tendency to provide a multitude of functions and high performance, some of commercially available image forming apparatuses such as copiers are provided with a plurality of copy sheet feeder units arranged stage by stage which are capable of continuously feeding a plurality of sheets different in size and material. In recent years, as the copiers have more functions, there has been a demand from users' side for a function of automatically inserting a tab sheet (sheet provided with a tab at its side edge) in a specified position (between specified pages) of a copied material.
Even in the existing copier, both tab sheets and normal copy sheets (hereafter, "normal sheets") can be fed if they are set in a plurality of copy sheet feeder units. However, these copiers are not provided with a special copying mode, it is impossible to, in a normal copying mode, perform a copying operation while automatically inserting the tab sheets in specified positions.
If the existing copier is provided with a special copying mode for inserting tab sheets, and a sheet to be fed is switched from the normal sheet to the tab sheet in a specified tab sheet inserting position (insertion page) in this mode, the tab sheets can be automatically inserted. However, considering processings during the transport of the tab sheet after being fed from the feeder unit and after being discharged, it is difficult to suitably produce a copied material with the inserted tab sheets simply by switching the sheets to be fed.
Specifically, since a tab projects from a longitudinal side edge of the tab sheet, it is necessary to set the tab sheets with their longer sides having no tab facing forward and to feed the tab sheets with their longitudinal direction normal to their feed direction in consideration of positioning of the leading ends of the tab sheets by the registration rollers, sheet alignment and sheet processing such as punching and stapling applied after the copied sheets are discharged onto the discharge tray. It is also necessary to reverse the transport direction of the tab sheet so that the longer side thereof having a tab comes forward after the tab sheet passes the fixing device. However, the prior art copiers are not provided with a function of reversing the transport direction of the tab sheet, and this leads to problems such as an erroneous feed timing, an error feeding such as oblique feeding, misalignment of the copied material in which the tab sheets are inserted, and a sheet processing error.